Hey there! Congrats on the new watch, and thanks for stopping by to ask!
I had kind of a tough time digging up network details on this watch, but as best I can tell if the specs listed here: DZ09 Smartwatch - SmartWatch Specifications are accurate, then the watch will only be able to take advantage of one of T-Mobile's LTE bands (1900 MHz), so your experience using it on the network with a wearable plan and a SIM card could vary pretty drastically depending on where you're located and how strong the 1900 presence is in that area. You should still be able to use it with your phone via BT, but as far as a standalone device is concerned, this is an area where we'd want to be straightforward by referring you to our Non-T-Mobile phone & software disclaimer.
If you have any friends or family that are using a physical SIM (instead of an eSIM) with a wearable device on the T-Mobile network, it might not be a bad idea to see how they'd feel about letting you do a little bit of a test drive/trial run! If you decide to give it a shot, you may want to check out our Bring your own device setup guide and see if there's a menu in the watch where you can input the BYOD T-Mobile data & APN settings!